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Copyright Pearson Prentice Hall

Biology

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36–2 The Muscular System


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The function of the muscular system is movement.

More than 40% of the mass of the average human body is muscle.


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Types of Muscle Tissue


What are the three types of muscle tissue?


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There are three different types of muscle tissue: • skeletal • smooth • cardiac


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Skeletal Muscles

Skeletal muscles:

• are usually attached to bones.

• are responsible for voluntary movements.

• have many nuclei.

• are sometimes called striated muscles.


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Smooth Muscles

Smooth muscles:

• are usually not under voluntary control.

• are spindle-shaped.

• have one nucleus.

• are not striated.

• are found in many internal organs and blood vessels.


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Most smooth muscle cells can function without nervous stimulation.

They are connected by gap junctions that allow electrical impulses to travel among muscle cells.


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Cardiac Muscle

Cardiac muscle:

• is only found in the heart.

• is striated.

• may have one or two nuclei.

Cardiac muscle cells are connected to each other by gap junctions.


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Muscle Contraction

Muscle Contraction

The fibers in skeletal muscles are composed of smaller structures called myofibrils.

Each myofibril has smaller structures called filaments.

The thick filaments contain a protein called myosin.

The thin filaments contain a protein called actin.


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Muscle Contraction

Filaments are arranged along the muscle fiber in units called sarcomeres.

Sarcomeres are separated by regions called Z lines.

When a muscle is relaxed, there are no thin filaments in the center of a sarcomere.


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Muscle Contraction

Skeletal muscle

Bundle of muscle fibers

Muscle fiber (cell)

Myofibril

Z line

Sarcomere

Myosin

Actin


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Muscle Contraction

How do muscles contract?


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Muscle Contraction

A muscle contracts when the thin filaments in the muscle fiber slide over the thick filaments.

This process is called the sliding filament model of muscle contraction.

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Muscle Contraction

During muscle contraction, the actin filaments slide over the myosin filaments, decreasing the distance between the Z lines.


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Muscle Contraction

Binding sites Myosin

Cross-bridge

Actin

Movement of Actin Filament


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Muscle Contraction

During muscle contraction, the head of a myosin filament attaches to a binding site on actin, forming a cross-bridge.


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Muscle Contraction

Powered by ATP, the myosin cross-bridge changes shape and pulls the actin filament toward the center of the sarcomere.


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Muscle Contraction

The cross-bridge is broken, the myosin binds to another site on the actin filament, and the cycle begins again.


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Muscle Contraction

When many myosin cross-bridges change shape in a fraction of a second, the muscle fiber shortens with considerable force.

The energy for muscle contraction is supplied by ATP.


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Control of Muscle Contraction


Impulses from motor neurons control the contraction of skeletal muscle fibers.

A neuromuscular junction is the point of contact between a motor neuron and a skeletal muscle cell.


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Vesicles in the axon terminals of the motor neuron release a neurotransmitter called acetylcholine.

Acetylcholine produce an impulse in the cell membrane of the muscle fiber.


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The impulse causes the release of calcium ions within the fiber.

The calcium ions affect regulatory proteins that allow actin and myosin filaments to interact.


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A muscle cell remains contracted until the release of acetylcholine stops and an enzyme produced at the axon terminal destroys any remaining acetylcholine.

Then, the cell pumps calcium ions back into storage, the cross-bridges stop forming, and contraction ends.


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How Muscles and Bones Interact


Skeletal muscles are joined to bones by tendons.

Tendons pull on the bones so they work like levers.

The joint functions as a fulcrum.

The muscles provide the force to move the lever.


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Opposing Muscles Contract and Relax


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Opposing Muscles Contract and Relax


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A controlled movement requires contraction by both muscles.


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Exercise and Health

Exercise and Health

Why is exercise important?


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Exercise and Health

Regular exercise is important in maintaining muscular strength and flexibility.


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Exercise and Health

Aerobic exercises help the body’s systems to become more efficient.

Resistance exercises increase muscle size and strength.

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36–2

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36–2

Skeletal muscles are joined to bones by tough connective tissue called

a. ligaments.

b. tendons.

c. filaments.

d. bursae.

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36–2

Muscle cells that are large, have many nuclei, and striations are

a. skeletal muscle cells.

b. smooth muscle cells.

c. cardiac muscle cells.

d. involuntary muscle cells.

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36–2

Muscle that is found in the walls of blood vessels and intestines is

a. skeletal muscle.

b. smooth muscle.

c. cardiac muscle.

d. striated muscle.

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36–2

The filaments found in skeletal muscle cells are

a. actin and myosin.

b. myosin and myofibrils.

c. actin and Z lines.

d. actin and sarcomeres.

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36–2

The type of muscle found only in the heart is

a. skeletal muscle.

b. striated muscle.

c. cardiac muscle.

d. smooth muscle.

END OF SECTION

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